Antimatter: Physicists Produce First Anti-Atom

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intrigued by the possibility that the driver is not within the climate system itself, but is instead the sun. "I think there's increasing evidence of solar variability playing a signif- icant role on century and millennia1 time scales," says paleoclimate modeler Thomas Crowley of Texas A & M University. Supporters of this idea point to a variety of evidence suggesting that solar activity var- ies on a time scale similar to their climate oscillations. There is, for example, an ap- proximate 2700-year cycle in salt deposition recorded in a now-dry saline arm of the ocean. The cycle is so old-more than 250 million years-that the sun is about the only thing whose behavior has been consistent enough over time to have caused it. If so, then the climate variations that caused the iceberg surges might also date back that far. Still, says Alley, "I don't believe that any- one has a convincing story" about what's caw- ing the oscillations. To convince him that the sun is indeed behind them, Alley wants to see variations in a uniquely solar signature in the ice cores. An approximate 2500-year oscillation has been reported in the amount Physicists Produce First Antiatom European physicists revealed their first glimpse into the shadowy world of antimat- ter last week, with the announcement by the CERN particle physics center in Geneva that researchers there had created the first few antiatom. An international team used CERN's Low-Energy Antiproton Ring (LEAR) to create a total of 11 atoms of antihydrogen, the simplest complete atom in the antiworld. "The CERN measurements are a milestone in the progress that physicists have made in producing exotic atoms," says theorist Stan Brodsky of the Stanford Linear Accelerator Center (SLAC) in California. Physicists are intensely excited at the prospect of being able to study this entirely new atom, because it will provide a funda- mental test for physicists' understanding of matter. "Astrophysicists tell us that in the big bane we should have antimatter as well as " matter, but ... in space they do not find anti- matter, at least not in the amounts they see matter," says Walter Oelert of the Institute for Nuclear Physics, part of the German Na- tional Researchcenter at Jiilich, who led the LEAR team. "A principal question for us is 'why is the amount of matter and antimatter different!' " says Oelert. Antimatter is the mirror image of the matter that makes up our world. Its existence was suggested by British theorist Paul Dirac in 193 1, whose famous quantum equation postulated the existence of an antiparticle counterpart of the electron. Dirac theorized that this particle, dubbed the positron, would have the same mass as the electron but oppo- site charge, and that the two would annihi- late on contact. The positron was found 2 years later, by Carl Anderson of the California Institute of Technology, while antiprotons were spotted at the University of California, Berkeley, in 1955. But until now, nobody has been able to combine a positron and an antiproton to cre- ate antihydrogen, because they are usually created in violent collisions, and slowing them down or closely matching their speeds is far from easy. In 1992, however, Brodsky and colleagues Charles Munger and Ivan Schmidt suggested a possible method for making antihydrogen. Oelert and his team, which includes scien- tists from the University of Genoa in Italy and other German universities, were the first to get it to work last September. The team directed a jet of xenon gas across the path of LEAR's antiproton beam. Very occasionally an antiproton is scattered by the positive charge of a xenon nucleus, converting part of its energy into an electron-positron pair. In a small fraction of these cases. the ~ositron's velocity was sufficiently close to that of the of carbon-14 in tree rings, which could be re- flecting the sun's influence on carbon-14 pro- duction in the atmosphere, but changes in the amount of carbon-14 returned from the deep ocean by varying currents could mimic the effects of solar variations. So Alley looks to ice core analyses of beryllium-10, an isotope whose abundance is solely dependent on so- lar activitv. A search for bervllium-10 varia- tions is under way, another factor that will keep climatologists' interest in the recent past high even as they try to predict the future. -Richard A. Kerr good agreement, better than we ever expect- ed," says Oelert. It will be a while, however, before physi- cists can start studying these intriguing new creatures, because they zip through the appa- ratus at close to the speed of light. ''This is a problem," says fellow LEAR scientist John Eades. But physicists are very keen to solve it: All of the currentlv acce~ted theories for the four fundamental forces of nature depend on an absolute svmmetrv between matter and antimatter. Atoms of antihydrogen should emit light at exactly the same frequencies as hydrogen; any differences would be a huge iolt to the theories. "If we found a tinv dis- crepancy, that would have a tremendous in- fluknce on the way we look at the uni- verse and the way we interpret the big bang," says Eades. Although it might provide some clues to why antimatter seems to be in short supply in the uni- verse, "one would have to change es- sentially all of our ideas about the way the universe works and is constructed." Eades notes. Finding a break in the sym- metry, he adds, "would shed new dark on everything." Brodsky's colleague Munger will start a new experiment at SLAC later Forge for antimatter. CERN's Low-Energy Antipro- this year that attempt spec- ton Ring made the first atoms of antihydrogen. troscopy on fast antihydrogen atoms produced in a similar way to Oelert's. scattered antiproton for the two particles to Other researchers are taking a different combine and create an antihydrogen atom. route. "It's only a matter of time till slow The Drocess of combination is so rare that antihvdroeen will be available." ~redicts prodicing 1 1 antihydrogen atoms required ~arvArd finiversity's Gerald ~ a l i r i i s e . His 5 x lo1* anti~rotons. team h o ~ e s to ~roduce antihvdroeen usine , - - Initially, the antihydrogen atoms kept trapped antiprotons and separately trapped moving along with the antiproton beam, but positrons, which would then be combined in as they have no net charge they evaded the a third trap. magnets that bend the beam around LEAR's While they wait for these new experi- running track-shaped path. So at the first ments, physicists find themselves confront- bend the atoms exited through a window ing a different problem: dismissing popular into a custom-built silicon detector. Here, reports that antimatter drives will soon be each antihydrogen atom broke up, only 40 powering spaceships to the stars. Says billionths of a second after its creation, and Brodsky: "Rocket engines burning antimat- the fragments produced a signal that enabled ter fuel will remain in the realm of fiction." the researchers to identifv the antihvdroeen -Andrew Watson , " atoms. "We should have expected to see nine events and we saw 11 k 2, which is extremely Andrew Watson is a writer in Norwich, U.K. SCIENCE VOL. 271 12 JANUARY 1996


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